The invention relates to a gas diffusion electrode, especially to an oxygen-consuming electrode for the reduction of oxygen under alkaline conditions, especially suitable for use in chloralkali electrolysis, with a new specific catalyst morphology and to an electrolysis apparatus. The invention further relates to a production process for the oxygen-consuming electrode and to the use thereof in chloralkali electrolysis or fuel cell technology.
The invention proceeds from oxygen-consuming electrodes known per se, which are configured as gas diffusion electrodes and typically comprise an electrically conductive carrier and a gas diffusion layer and a catalytically active component.
Oxygen-consuming electrodes, referred to hereinafter as OCEs, are one form of gas diffusion electrodes. Gas diffusion electrodes are electrodes in which the three states of matter—solid, liquid and gaseous—are in contact with one another, and the solid, electron-conducting catalyst catalyses an electrochemical reaction between the liquid and gaseous phases. The solid catalyst is typically pressed to a porous film, typically with a thickness of more than 200 μm.
Various proposals for operation of the oxygen-consuming electrodes in electrolysis cells on the industrial scale are known in principle from the prior art. The basic idea is to replace the hydrogen-evolving cathode of the electrolysis (for example in the chloralkali electrolysis) with the oxygen-consuming electrode (cathode). An overview of the possible cell designs and solutions can be found in the publication by Moussallem et al. “Chlor-Alkali Electrolysis with Oxygen Depolarized Cathodes: History, Present Status and Future Prospects”, J. Appl. Electrochem. 38 (2008) 1177-1194.
The oxygen-consuming electrode—also referred to hereinafter as OCE for short—has to meet a series of fundamental requirements to be useable in industrial electrolysers. For instance, the catalyst and all other materials used have to be chemically stable toward approx. 32% by weight sodium hydroxide solution and toward pure oxygen at a temperature of typically 80-90° C. Similarly, a high degree of mechanical stability is required, such that the electrodes can be installed and operated in electrolysers with a size typically more than 2 m2 in area (industrial scale). Further properties are: high electrical conductivity, low layer thickness, high internal surface area and high electrochemical activity of the electrocatalyst. Suitable hydrophobic and hydrophilic pores and a corresponding pore structure for conduction of gas and electrolyte are likewise necessary, as is such imperviosity that gas and liquid space remain separate from one another. The long-term stability and low production costs are further particular requirements on an industrially useable oxygen-consuming electrode.
A further direction of development for utilization of OCE technology in chloralkali electrolysis is that of zero-gap technology. In this case, the OCE is in direct contact with the ion exchange membrane, which separates the anode space from the cathode space in the electrolysis cell. No gap for sodium hydroxide solution is present here. This arrangement is typically also employed in fuel cell technology. A disadvantage here is that sodium hydroxide solution which forms has to be passed through the OCE to the gas side, and then flows downward on the OCE. In the course of this, there must not be any blockage of the pores in the OCE by the sodium hydroxide solution, or crystallization of sodium hydroxide solution in the pores. It has been found that very high sodium hydroxide solution concentrations can also arise here, in which case the ion exchange membrane does not have long-term stability to these high concentrations (Lipp et al., J. Appl. Electrochem. 35 (2005)1015—Los Alamos National Laboratory “Peroxide formation during chlor-alkali electrolysis with carbon-based ODC”).
An important prerequisite for the operation of gas diffusion electrodes is that both the liquid and gaseous phases may be present at the same time in the pore system of the electrode. How this is to be achieved is shown by the Young-Laplace equation:
  p  =            2      ⁢      σcos      ⁢                          ⁢      θ        r  
The gas pressure p is thus related to the liquid in the pore system via the pore radius r, the surface tension σ of the liquid and the wetting angle Θ. However, this equation should be understood merely as a guide because too many parameters are unknown or difficult to determine:                For the surface tension, the difference of the surface tension of the solid and of the liquid has to be considered. The surface tension of catalysts, for example platinum on carbon or silver, however, are barely measurable.        The wetting angle can be determined on a flat surface. A single pore, in contrast, cannot be examined since the pore system of the entire electrode would be determined in this case.        The wetting angle also changes under the influence of the electrical field and the temperature; neither can be measured within the electrode.        
To create gas and liquid spaces in an OCE, it is necessary to generate pores which have different pore radii or different surface tensions. In addition to the wetting properties, the OCEs must have a good electrical conductivity, in order that the electrons can be transported with a minimum ohmic resistance.
In chloralkali electrolysis in the finite gap arrangement, for example, an OCE separates an electrolyte space from a gas space. In this case, as described above, gas must not pass from the gas space into the electrolyte space, nor electrolyte from the electrolyte space into the gas space. In industrial electrolysers, the oxygen-consuming cathode should withstand the hydrostatic pressure which exists at the base of the industrial electrolysis cell, for example of 170 mbar. Since a gas diffusion electrode has a pore system, a small amount of liquid always passes into the gas space, and gas into the liquid space. The amount depends on the construction of the cell of the electrolyser. The OCE should be impervious at a pressure difference between the gas space and the liquid space in the range of 10-60 mbar. What is meant here by “impervious” is that passage of gas bubbles into the electrolyte space is observable with the naked eye. “Liquid-impervious” means that an amount of liquid of not more than 10 g/(h*cm2) passes through the OCE (where g represents the mass of liquid, h represents one hour and cm2 represents the geometric electrode surface area). When, however, too much liquid passes through the OCE, it can flow downward only on the side facing the gas side. In this case, a liquid film can form, which hinders the access of gas to the OCE and thus has an extreme adverse effect on the performance of the OCE (oxygen under-supply). When too much gas enters the electrolyte space, it has to be possible to conduct the gas bubbles out of the electrolyte space. In each case, the gas bubbles screen off part of the electrode area and of the membrane area, which leads to a current density shift and hence, in galvanostatic operation of the cell, to a local increase in current density and to an undesirable increase in cell voltage over the cell.
An alternative possibility is that of using sintered electrodes. In this case, it is possible, for example, to use three different particle sizes in different layers of the OCE. For instance, a top layer may consist of fine material, a working layer of different fractions, and a gas conduction layer of coarse material (DE 1219553).
A disadvantage of these electrodes is that the electrodes are comparatively thick and heavy—typical thicknesses are approx. 2 mm. The individual layers must be very thin but fault-free. The cost of metal for this kind of electrode is comparatively high, and it has not been possible to produce the electrodes in continuous production processes.
A further disadvantage of this kind of gas diffusion electrodes is that they are very sensitive to pressure variations and cannot be used, for example, in industrial electrolysers since, owing to the construction height, the electrolyte here has a high hydrostatic pressure at the base of an electrolysis cell, which acts on the gas diffusion electrode and thus floods the pore system.
Such electrodes have been produced by scattering application and subsequent sintering or hot pressing. In order to produce multilayer electrodes, a fine material was thus first scattered into a template and smoothed. Subsequently, the other materials were applied in layers one on top of another and then pressed. The production was not only error-prone but also time-consuming and difficult to automate.
EP797265 (Degussa) describes a gas diffusion electrode and a production process for a gas diffusion electrode, which leads to a bimodal pore distribution in the electrode layer. In this case, a catalyst is dispersed with a proton-conducting ionomer. The total porosity of the electrode is 40 to 75% and is composed of small pores with mean diameters up to 500 nm and large pores with mean diameters of 1000 to 2000 nm. The small pores are formed as the solvents vaporize after spraying of the coating dispersion onto a hot membrane. The large pores form as a pore former added beforehand is decomposed or leached out. The mean diameter of the pores can therefore be influenced by the particle size of the pore former used. The bimodal pore distribution is intended to bring about improvement of mass transfer in the electrode layer. Through the macropores, the reaction gas can rapidly pass deep into the electrode layer, and the water of reaction formed can be removed. The small pores then undertake the transport in the ion-conducting polymer as far as the catalyst particles. The distances to be covered here are only short, and so the slowed transport in the small pores does not significantly impair the performance of the electrode. A distinct improvement in transport in the electrode layer over conventional coatings is observed only at total porosities of more than 40%. The supply of the electrocatalyst with the reaction media increases with rising porosity. However, the amount of available electrocatalyst and of the ionomer in the coating decreases with rising porosity. Thus, the attachment of the catalyst to the ionomer and the ionic conductivity of the coating deteriorate with rising porosity, and so the performance data of the electrode layer deteriorates again at porosities above 75%.
This applies to fuel cell electrodes in which the electrocatalyst is dispersed in a proton-conducting polymer.
The document U.S. Pat. No. 6,503,655 describes a hydrophobic gas diffusion electrode with a smooth surface for use in the PEM fuel cell which has pore diameters of 10 to 10 000 nm. The permeability of the electrodes for nitrogen should be greater than >10−6 m2/s at standard pressure, preferably >10−5 m2/s. For this purpose, the largest pores should have a diameter of more than 100 nm; the diameter should preferably be 500 to 10 000 nm. Also important is a hydrophobic character of the electrodes. This namely prevents water formed in the electrochemical reaction between hydrogen and oxygen from collecting in the pores and blocking them. In order to meet the requirements mentioned, modified carbon papers are used in gas diffusion electrodes, i.e. carbon papers whose surface density has been increased with carbon black or graphite. However, these materials are inadequate with regard to surface smoothness and pore size. U.S. Pat. No. 6,503,655 does not make any statement regarding pore volume or porosity.